1. Field of the Invention
This invention relates to an enhanced purification method by which a high-purity metallic indium feed with a purity of about 99.99% (4N) is further purified to give metallic indium with a purity of about 99.9999% (6N) or higher and which is also applicable for such enhanced purification of antimony, zinc, tellurium, magnesium, cadmium, bismuth and silver (which are hereunder referred to as “similar metals”). The invention also relates to an apparatus for purification that is used to implement the method.
2. Background Information
Indium is generally produced as a minor amount component of zinc concentrates, so in zinc metallurgy, it is recovered either as a flue cinder or as a concentrate obtained in an intermediate step such as an electrowinning of zinc. In recent years, indium is also recovered in pure form from waste compound semiconductors. To purify the indium feed, three methods are commonly used and they are electrolysis, vacuum distillation and zoning.
The metallic indium obtained by electrolysis or vacuum distillation is about 99.99% pure and contains at least 0.5 ppm each of impurities such as Si, Fe, Ni, Cu, Ga and Pb. The purification from waste compound semiconductors has the problem that large equipment and prolonged time are needed to separate and recover indium.
In the zone purification method, the purified indium mass has to be cut and there is a potential hazard of contamination; hence, the purification process inevitably suffers from a limited throughput and a lowered yield. In addition, when the purified indium is cast into an ingot, impurities may enter during casting to cause contamination.
With a view to solving these problems, researchers of Dowa Mining Co., Ltd. of Tokyo, Japan, including one of the present inventors, previously developed an improved technology for purifying indium to a purity of at least 99.9999% by vacuum distillation and proposed it in Japanese Patent Application No. 8-294430 (now matured into JP 10-121163A, hereinafter referred to as “JP-163”). As it turned out, this technology had the problem that purification became more difficult as the difference between the vapor pressures of the metal of interest and impurity elements decreased. Hence, it was desired to develop a purification technology that was capable of producing indium of higher purity with higher efficiency and which was also applicable to the similar metals mentioned above.
The purifying method of JP-163 could be carried out with no problems when it was practiced in a laboratory scale for a relatively short time period. However, the inventors encountered a lot of trouble when they conducted tests of a longer duration. Hence, they recognized that it would be difficult to carry out the method of JP-163 continuously for a very long time period, particularly when it should be conducted in a commercialized scale.
One of the problems was that the production of 6N-purity indium could not be maintained for a long period because of causing an abnormal increase in the Si content of the final product.
Another problem was that the outer quartz cylinder 20 (see FIG. 2) was distorted or deformed when it was heated to a high temperature for a long period.
A further problem was that an unknown white powdered material was formed in the inner quartz cylinder 21 (see FIG. 2) when subjected to an enhanced temperature for a prolonged time and it accumulated on top of the purified indium collected in the recovery mold 23.
In order to solve the problems mentioned above, the inventors conducted intensive studies and finally succeeded in solving all of them. After repeating a lot of trials and errors, the inventors solved the problems one by one and finally attained the present invention.
As regards the first problem, it was considered that at an enhanced temperature, vaporized indium attacked the inner wall of the quartz cylinder and when the temperature became too high, deterioration of the quartz cylinder occurred and recontamination of the purified indium took place. To avoid the reaction of indium vapor and the quartz cylinder, the inventors decided to use an inner tube made of graphite, instead of the quartz inner cylinder. As a result, they were successful in stabilizing the purifying temperature and highly improving the rate of purification, as compared with the method of JP-163.
The second problem came from the softening of the quartz by heat and the pressure applied thereon due to the pressure difference between the outside (air) and the inside (vacuum) atmospheres of the outer quartz cylinder 20. Accordingly, it seemed necessary to drastically change the structure of the purifying apparatus. Hereupon, in order to prevent the outer quartz cylinder 20 from softening and deforming under high temperatures and the pressure difference between air and high vacuum atmospheres, it was found necessary to make a quartz cylinder having a very thick wall. It turned out to be very expensive to make a large scale purifying apparatus of a quartz cylinder having such a thick wall. Moreover, such a big apparatus would be difficult to handle because of increased weight. In addition, it turned out that even if the second problem was solved by construction of a thick wall outer quartz cylinder, the above-described first problem could not be solved.
Hereupon, the present inventors gave up using an outer quartz cylinder and instead made a new purifying apparatus having a rigid shell (also serving as an outer tube) that would not be deformed due to the pressure difference under a high temperature condition. The inside of applicants' furnace is entirely a vacuum, and heaters are provided fixed to the inner wall of the rigid shell which serves as an outer tube of a purifying apparatus. These heaters (6 and 7 in FIG. 1) can directly heat the inner graphite tube (3 in FIG. 1) and its whole contents.
As regards a third problem, the inventors considered that the white powdered material was produced by the reaction of vaporized indium and the inner wall of the quartz cylinder 21. It was confirmed by X-ray diffraction analysis that the white powdered material was a mixture of In and SiO2.
Thus, according to the present invention, the reconstruction of a purifying apparatus was made by integration of the electric furnace 18 (shown in FIG. 2), the outer quartz cylinder 20 (shown in FIG. 2), the inner quartz cylinder 21 (shown in FIG. 2) together with all the other members contained therein into a one-body assembly. Namely, in the purifying apparatus of the present invention, upper heaters 6 and lower heaters 7 are installed in a vacuum atmosphere between the outer tube 1 and the inner tube 3, each shown in FIG. 1.
By employing this structure, the problem of distortion or deformation of the outer quartz cylinder 20 due to high temperature and the pressure difference discussed hereinabove were solved. In the apparatus of JP-163, the electric furnace 18 (see FIG. 2) was separately used by placing it surrounding the quartz outer cylinder 20, and heat was applied by an external source (electric furnace 18) through the air atmosphere. In contrast thereto, in the apparatus of the present invention, the electric furnace and the inner tube are integrated into one body.
According to JP-163, a metal feed was heated at a temperature of 1000° C. or higher in a first thermal purification step, but no special temperature control was made for conducting a second thermal purification step. In a laboratory scale production of high purity indium, this was satisfactory from the viewpoints of both desired purity and production efficiency. Thus, no problem was recognized. In the practice of an enlarged scale, however, it was found necessary to enhance the temperature in the first thermal purification step to a range of 1100° C. to 1300° C. to obtain improved production efficiency. It was also found necessary to closely control the temperature within the range of 900° C. to 1000° C. in the second thermal purification step so as to constantly obtain the desired high purity product.
In the purifying apparatus of the present invention, heaters made of graphite are employed to avoid the reaction with indium. The positioning of in-furnace members of the purifying apparatus is made in such a manner that the assembling of all the members is made in advance at a place outside of and below, but not directly below the furnace. Then, the already assembled members are automatically moved in a horizontal direction until they reach a position directly below the furnace, where they should stand. Then the furnace is automatically lowered from an upper level position until it reaches the position where the assembled members stand, and the furnace is fixed there surrounding the already assembled in-furnace members of the purifying apparatus.
In the apparatus of the present invention, the outer periphery of the furnace is cooled with water which flows in a water jacket provided within the shell generally made of stainless steel. Therefore, the temperature drop after the finish of the purifying operation is rapid (about one half of the time in the case of air cooling). Namely, the operation time required is about one half that required in the case of JP-163. For example, in the case of water cooling, it took 4 hours before the furnace temperature reached the ordinary temperature after finishing the operation, though in the case of air cooling (as in JP-163), it took 8 hours to attain the same thing. Thus, purifying time can be greatly shortened.
As members to be used in a furnace, carbon heaters (6 and 7), carbon fiber heat-insulating material (17), as well as an inner carbon tube (3) are used in order to avoid contamination from the in-furnace member materials as much as possible. As a result of changing the heating manner from an external heating type to the in-furnace heating system, temperature-monitoring points were also changed from the points outside the outer quartz cylinder 21 in FIG. 2 to the points between the carbon heaters (6 or 7 in FIG. 1) and the inner carbon tube 3 (in FIG. 1). By this change the closer control of the purifying temperature has become possible. Moreover, in the case of the conventional furnace shown in FIG. 2, the open air entered the space surrounding the outer quartz cylinder 21, where heat convection occurred and it was difficult to make correct temperature control because of the influence of ascending current on the measuring points of thermocouples.